This invention relates to a device for automatically reducing excess play in a movement, such as the movement occurring in brake shoes, brake systems, or the like, in which the device is incorporated.
The problem posed by an excessive amount of play in a movement and related controls, such as in brake shoes, is a well known one to the experts, and considerable effort has gone into its study, with a view to preventing or at least accommodating it.
Whilst brakes are not the only type of application whereto this invention is directed, specific reference will be made herein to them, inasmuch as they represent the best known and more frequently encountered of cases, and accordingly the most suitable example from a didactic standpoint.
In a brake including shoes and linings, the play or free movement in question corresponds to the amount of idle movement of the shoes before they engage a wheel to be braked, either through a drum or disk applicator; as the wear of the frictional linings progresses, the play and accordingly the idle movement also increases. This drawback has various effects, depending on the brake design and the particular utilization thereof. As far as this problem is concerned, all of the following shoe, or band, or disk brakes fall into a common class: braking systems intended for use with capstans, hoists, overhead traveling cranes, elevators, cableways, chairlifts, conveyors, etc.; and it is to such installations that this invention is particularly directed, albeit not exclusively to them, as mentioned already. In FIG. 1 of the accompanying drawings, there is shown a typical brake layout for installations such as the ones mentioned above, reference being made to that diagram to further clarify the terms of the problem. The reference numeral 1 designates generally the brake assembly and related controls, 2 designates the brake wheel or pulley, adapted for frictional engagement between two opposite brake shoes 3 having frictional linings 4 facing the contacting surface of the wheel. The shoes are both carried and moved by levers 5 which are connected to articulated rods 6,7,8, the latter whereof is positively actuated by a spring 9 and released through an electromagnetic servo 10, as indicated by the arrows 11 and 12. Between the frictional linings 4 of the shoes and the contacting surfaces of the wheel 2, there is a gap or clearance 13, which has originally a minimum preset value that increases thereafter continuously as the wear of the lining 4 progresses. The increasing gap or play 13 results in all the levers and articulated rods operating in varied and no longer optimal positions, until the electromagnet reaches its stroke limit, which prevents any further utilization of the braking system. However, already before this extreme condition of no utilization is reached, the spring will relax, which causes the system to operate at a reduced power. In order to utilize the entire thickness or depth of the lining 4, without restoring the initial conditions through suitable re-adjustments, it would be necessary to provide much longer levers and rods, a much more powerful spring, and an electromagnet of proportionally larger dimensions, or alternatively reduce the link 6 by an amount proportional to the play increase. Besides the considerable and unacceptable cost increase involved, that approach is inadequate, owing mainly to bulk considerations as the experts will readily recognize. Thus, it has become necessary, from the very beginning, to provide a means of adjusting the shoes on their related levers, e.g. to restore the increased play 13 to its initial value. However, such an approach is scarcely a desirable one, because it involves an appreciably high added maintenance cost, and even more so because any occasional delay or higher wear rate could results in poor operational effectiveness and troublesome malfunctions. Understandably, attempts have been made to reduce the excess play automatically without discontinuing the operation. Since it is not convenient to adjust the shoes on their related levers, provision has been made for approaching the two levers 5, in their inoperative positions, which carry the shoes, such as by shortening the link 6, which is suitably threaded and threadably engaged with a sleeve, in a manner known per se. The threading movement is in this instance controlled upon occasion by the travel or stroke excess of the electromagnet or of the lever 8. But this is only possible and convenient in those cases where room is available for such attachments, which cannot fail to increase the overall bulk, albeit moderately. In a majority of cases, this is not possible, however, or is undesirable. Thus, the need is felt for a solution to this important and frequently encountered problem which is universally acceptable.